Recently portable and cordless tendency in electronic appliances for general use has been rapidly progressing. Accordingly, a demand for a small-size, light-weight secondary battery having a high energy density, used for charge and discharge power supply, has been increasing. From this point of view, non-aqueous batteries, and particularly lithium secondary batteries are anticipated as batteries having high voltage and energy density and the development of these batteries is urged.
Conventionally, manganese dioxide, vanadium pentaoxide, titanium disulfide and the like have been used as a cathode active material of the lithium secondary batteries. Such a battery comprises a cathode of these materials, a lithium anode and an organic electrolyte and charge and discharge are repeated. However, in a secondary battery employing a lithium metal anode, the problems of internal short circuit caused by dendritic lithium generated upon charging or side reactions between an active material and an electrolyte are a barrier to developing secondary batteries. Further, there has not yet been found a secondary battery which satisfies the high rate charge and discharge property and the over discharge property. Furthermore, the safety of the lithium batteries has been severely pointed out and in battery systems employing a lithium metal or a lithium alloy therein as an anode, the safety is very difficult to ensure.
Recently, a new type of negative electrode has attracted interest, in which the intercalating reaction of layered compounds is utilized for solving the above problems. Particularly graphite compound incorporating lithium has been investigated as an anode material for the attracted secondary lithium battery.
However, graphite compound incorporating lithium therein is very unstable and in the case of using natural or artificial graphite with high crystallinity as an anode, the battery lacks the cycle stability and the capacity thereof is low. Further since decomposition of the electrolyte takes place with charge and discharge reaction, graphite compound cannot substitute for a lithium anode.
Lately it has been found that lithium-doped materials of pseudo graphite materials, which are provided with more or less turbostratic structure and low crystallinity, obtained by carbonization of a variety of hydrocarbon or polymeric materials, are effective as an anode and can receive relatively wide application, and further have excellent stability in a battery. Accordingly many researches on small-sized, light-weight batteries with the use of these materials have been made.
On the other hand, as more carbon materials are used as an anode, it is proposed that such Li-contained compounds having higher voltage as LiCoO.sub.2 or LiMn.sub.2 O.sub.4 or composite oxide in which a part of Co and Mn is displaced by other elements such as, for example, Fe, Co, Ni, Mn and so on are to be used as a cathode active material.
On testing some of the afore-mentioned pseudo graphite materials, a capacity of only 100-150 mAh/g carbon is obtained and also polarization of carbon electrode, accompanied with charge and discharge, is relatively large. Therefore, when these carbon anode materials are used in combination with a cathode of, for example, LiCoO.sub.2 and so on, it is disadvantageous to obtain relatively low voltage and it is difficult to obtain satisfactory capacity. In general, it has been reported that the upper limit of the amount of lithium chemically intercalated between the graphite layers corresponds to that which forms a graphite intercalation compound C.sub.6 Li (called a first stage) wherein lithium atom is inserted between six carbon atoms. In this case, the capacity is theoretically calculated to 372 mAh/g carbon. In contrast to this, the capacity of pseudo graphite material is lower than that of the above case. This is the reason why the pseudo graphite has an undeveloped layer structure or low crystallinity of graphite, so that it is not enough for lithium to intercalate between the layer structure. On the other hand, it has been reported that in the case of using a graphite material with high crystallinity as an anode, intercalation reaction of lithium is difficult to proceed due to the gas generated upon charging on the surface of the graphite electrode surface by the decomposition of an electrolyte. It is found that in spite of generating the gases, coke with relatively high crystallinity heat-treated at a high temperature gives relatively high capacity (200-250 mAh/g). However, due to the expansion and contraction of the graphite in the C axis direction, accompanied by the charge and discharge reaction and by such a volume variation resulting from high capacity, such a high crystalline graphite anode is swollen and thus results in poor cycle property. That is, there is a tendency that the low crystalline graphite is superior in the cycle property while the high crystalline graphite is superior in the capacity.